Most big retailers would like you to think they're always the best source for low prices and you should never shop anywhere else. We all know that's not true, but finding the best price can be tough. Fortunately, Yahoo Finance has done a bit of research and found some of the best and worst things to buy at several popular stores.

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Amazon.com
is a great for auto parts, diapers, gluten-free foods, and computer peripherals/accessories. Why? Amazon has an incredible auto part database that's easy to search and find what you need, you can get 30% off selected diapers through Amazon's Subscribe & Save program, there's a surprisingly large selection of gluten-free foods, and Amazon has always been a great source for technology with plenty of reviews to help you make your decision. I'd also add that they're great for ordering computer parts when you're building a machine. Often times their prices are as low or lower as Newegg's (although not always) and they have a much better return policy.

However, you should avoid groceries, hardware tools, home furnishings, and cleaning supplies. Why? Groceries only come in bulk, hardware tools and furniture are things you'll want to see first, and cleaning supplies are unusually expensive.

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Costco is a bad place to buy designer clothing, imported shrimp, sheets and towels, and 12 lb. crates of navel oranges. Imported shrimp is bad because environmental regulations are lax in the countries where it originates, huge crates of oranges are extremely unlikely to be finished (so only buy if you can split with friends ), clothing standards are lower at Costco so brand names might not be up to the quality you expect, and Target and Walmart are just better for sheets and towels thanks to a bigger selection at a smaller price.

However, Costco is a great place to fill up your car with heavily discounted gas, buy printer and computer supplies like ink cartridges, paper, and writable CDs and DVDs, and alcohol. Why? The gas is almost always cheaper than your local gas station, you generally want to buy printer and computer supplies in bulk and they're cheaper at Costco than most retailers, and Costco has an excellent selection of wine and liquor at very competitive prices.

Walmart is a terrible place to buy electronics, wood furniture, and jewelry. Jewelry at Walmart is generally of poor quality in order to keep prices low, their furniture is generally pretty crappy and unethically sourced, and electronics because Walmart staff just aren't particularly well-versed in what they're selling you. If you need any level of customer support, visit practically anywhere else for your gadgets.

On the other hand, Walmart is great for big discounts on brand-name groceries and snacks. You'll also find great prices on smartphones, should you know what you want and not need to ask a specialist any questions. Walmart's also been improving its sustainability record with coffee, offering fair trade brands at good prices. Lastly, you'll find the same commitment to laundry detergent with eco-friendly brands available on a budget.

Target offers a good selection of inexpensive designer cosmetics. It's also a good place to buy the Amazon Kindle should you want to try it out first. Target also has a great selection of inexpensive, eco-friendly cleaning products that go beyond just the laundry detergent that Walmart provides. Target has also made a concerted effort to add a solid offering of groceries in many of its stores. I've found, surprisingly, that they often have certain items I can't find anywhere else. While the selection isn't always as fantastic as a proper grocery store, it's still pretty comprehensive.

Target's pretty bad when it comes to furniture, however. Often times you can just get something better and cheaper at IKEA. That's not always the case—I found some great chairs at Target—but the quality is about the same as IKEA so you may want to compare prices before choosing one or the other. Exercise equipment is another thing you'll want to avoid. Target tends to trend towards the "as seen on TV"-type stuff, which is generally costly and far less effective than just paying for a gym membership. Movies and music are also bad, as Walmart and Amazon are pretty much always cheaper. Target also has a lot of products made with polyvinyl chloride (PVC), which often contain lead and pthalates—substances that have been linked to reproductive issues and release carcinogens when incinerated. In particular, shower curtains and some toys often contain PVC, so you may want to check a product's content before checking out.